N-well/P-well strap structures

ABSTRACT

Embodiments of N-well or P-well strap structures are disclosed with lower space requirements achieved by forming the strap on both sides of one or more floating polysilicon gate fingers.

BACKGROUND OF THE INVENTION

This relates to N-well or P-well strap structures for use in integratedcircuits.

N-well or P-well strap structures are typically used in integratedcircuits to tie a source line to a well region so as to assure that thevoltage in the well region is the same as the voltage at the sourceline.

FIGS. 1 and 2 depict a top view and a cross-section along lines 2-2 ofFIG. 1 of a typical integrated circuit structure 100 that includes anactive device and a well strap formed in a well in a semiconductorsubstrate 105 It will be understood that this structure may bereplicated multiple times in the integrated circuit. Structure 100includes source and drain regions 110, 120 formed in a well 130 with apolysilicon gate finger 140 formed on a dielectric layer (not shown) onthe surface of well 130. These elements will be recognized as forming aMOS transistor; but it is to be understood that the MOS transistor isonly illustrative of any active device. Following industry practice, thelength L of gate 140 is its shorter dimension.

Structure 100 further includes diffusion region 160 that makes ohmiccontact with well 130 and ohmic contacts (or taps) 115 to source region110, ohmic contacts 125 to drain region 120, and ohmic contacts 165 todiffusion region 160. The diffusion region 160 and its contacts or taps165 constitute the well strap. A shallow trench isolation (STI) region150 surrounds the active device and well strap.

Illustratively, the transistor is a PMOS transistor, source and drainregions are P-type, well 130 is an N-type well, and diffusion region 160is N-type. Alternatively, the transistor is an NMOS transistor, sourceand drain regions 110, 120 are N-type, and well 130 and diffusion region160 are P-type.

In certain prior art integrated circuits, the N-well or P-well strap isplaced so that it is directly abutting an active device such as the MOStransistor as shown in FIGS. 1 and 2. Further details concerning such animplementation of a N-well strap may be found in U.S. Pat. No. 7,586,147B2 for “Butted Source Contact and Well Strap,” which is incorporatedherein by reference. In alternative structures, the well strap may forma ring around the active device or group of active devices.

In certain other prior art integrated circuits, dummy polysilicon isplaced next to the device gates so as to control uniformity of criticaldimensions. In this case, the well strap is spaced apart from the activedevice. FIG. 3 is a top view of such a prior art integrated circuitstructure 300 including an active device and a well strap.Illustratively, structure 300 includes source and drain regions 310, 320formed in a well 330 with a polysilicon gate finger 340 formed on adielectric layer (not shown) on the surface of well 330. These elementswill be recognized as forming a MOS transistor; but it is to beunderstood that the MOS transistor is only illustrative of any activedevice. A substrate (not shown) similar to substrate 105 of FIG. 2underlies well 330.

Structure 300 further includes diffusion region 360 that makes ohmiccontact with well 330 and ohmic contacts (or taps) 315 to source region310, ohmic contacts 325 to drain region 320, and ohmic contacts 365 todiffusion region 360. The diffusion region 360 and its contacts or taps365 constitute the well strap. A STI region 350 surrounds the activedevice and the well strap. Again, the transistor can be a PMOStransistor with P-type source and drain regions 310, 320 and N-type well330 and diffusion region 360; or the transistor can be a NMOS transistorwith N-type source and drain regions 310, 320 and P-type well 330 anddiffusion region 360.

Structure 300 further comprises dummy polysilicon gate fingers 371, 372located on opposite sides of the active device above portions of the STIregion 350. As a result, the well strap is separated from the activedevice by at least one length of the dummy polysilicon finger.

In certain other prior art integrated circuits, double dummy polysiliconis placed next to active devices. FIG. 4 is a top view of such a priorart integrated circuit structure 400 including an active device and awell strap. Illustratively, structure 400 includes on the left-hand sidesource and drain regions 410, 420 formed in a well 430 with apolysilicon gate finger 440 formed on a dielectric layer (not shown) onthe surface of well 430. These elements will be recognized as forming afirst MOS transistor; but it will be understood that the MOS transistoris only illustrative of any active device. A second MOS transistor isformed on the right-hand side of FIG. 4 and includes the same elementsbearing the same numbers followed by the suffix A. Again, a substrate(not shown) similar to substrate 105 of FIG. 2 underlies well 430.

Structure 400 further includes diffusion region 460 that makes ohmiccontact with well 430 and ohmic contacts (or taps) 415 to source region410, ohmic contacts 425 to drain region 420, and ohmic contacts 465 todiffusion region 460. The diffusion region 460 and its contacts or taps465 constitute the well strap. A STI region 450 surrounds the activedevices and the well strap. Again, the transistor can be a PMOStransistor with P-type source and drain regions 410, 420 and N-type well430 and diffusion region 460; or the transistor can be a NMOS transistorwith N-type source and drain regions 410, 420 and P-type well 430 anddiffusion region 460.

Structure 400 further comprises dummy polysilicon gate fingers 471, 472,473, 474 with the first two fingers 471, 472 located above portions ofSTI region 450 between the well strap and the first transistor and thesecond two fingers 473, 474 being located above other portions of STIregions 450 between the well strap and the second transistor. As aresult, the well strap is separated from the active device by at leasttwo lengths of the dummy polysilicon fingers.

SUMMARY OF THE INVENTION

The use of increasing numbers of dummy polysilicon gate fingers toseparate the active device(s) from the well strap takes up considerableamount of space on the semiconductor substrate. The present invention isimproved N-well or P-well strap structures with lower spacerequirements. In illustrative embodiments, reduced space requirementsare achieved by forming the strap on both sides of one or more floatingpolysilicon gate fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention will beapparent to those of ordinary skill in the art in view of the followingDetailed Description in which:

FIGS. 1 and 2 are a top view and a cross-sectional view of a first wellstrap structure of the prior art;

FIG. 3 is a top view of a second prior art well strap structure;

FIG. 4 is a top view of a third prior art well strap structure;

FIGS. 5 and 6 are a top view and a cross-sectional view of a firstillustrative embodiment of the invention;

FIG. 7 is a top view of a second illustrative embodiment of theinvention;

FIG. 8 is a top view of a third illustrative embodiment of theinvention; and

FIG. 9 is a top view of a fourth illustrative embodiment of theinvention.

DETAILED DESCRIPTION

FIGS. 5 and 6 are a top view and a cross-sectional view of a firstillustrative embodiment of the invention. Structure 500 includes sourceand drain regions 510, 520 formed in a well 530 with a polysilicon gatefinger 540 formed on a dielectric layer (not shown) on the surface ofwell 530. These elements will be recognized as forming a MOS transistor;but it will be understood that the MOS transistor is only illustrativeof any active device that may be used in the practice of the invention.A second MOS transistor is formed on the right-hand side of FIG. 5 andincludes the same elements bearing the same numbers followed by thesuffix A. As shown in FIG. 6, a semiconductor substrate 605 underlieswell 530.

Structure 500 further includes diffusion regions 560, 562 that makeohmic contact with well 530, a floating polysilicon gate finger 580between diffusion regions 560, 562, and ohmic contacts (or taps) 515 tosource region 510, ohmic contacts 525 to drain region 520, and ohmiccontacts 565, 567 to diffusion regions 560, 562. The diffusion regions560, 562, and contacts or taps 565, 567 constitute the well strap. A STIregion 550 surrounds the active devices and the well strap. As shown inFIG. 5, taps 565 and taps 567 are on opposite sides of floating gatefinger 580. While two taps 565 and two taps 567 are shown, a single tap565 or 567 or more than two taps 565 or 567 may be used.

Structure 500 further comprises dummy polysilicon gate fingers 575, 576located on opposite sides of diffusion regions 560, 562 and aboveportions of STI region 550. As a result, the well strap is separatedfrom the active device by only one length of the dummy polysilicon gatefinger, thereby reducing the distance between the active device and thediffusion region 560 compared with the distance between the activedevice and the diffusion region 460 in the prior art structure of FIG.4.

To form structure 500, dopants of a first conductivity-type,illustratively N-type, are first implanted in a substrate 602 of asecond conductivity type, illustratively P-type, to form an N-type well530. STI region 550 is then formed in well 530. An insulating layer isthen formed on the surface of the well; and polysilicon gate fingers540, 540A, 575, 576, 580 are formed on the insulating layer. Lightlydoped drain regions are then formed in the well on each side of gates540, 540A; and sidewalls 542, 542A are then formed on the sides of gates540, 540A. The gates and sidewalls are then used as masks to control theimplantation of dopants during formation of the source and drain regionsand the diffusion regions. Illustratively P-type dopants are implantedon both sides of gates 540, 540A and sidewalls 542, 542A to form sourceregions 510, 510A and drain regions 520, 520A of the PMOS transistors;and N-type dopants are implanted on both sides of gate finger 580 toform diffusion regions 560, 562. Because the gates and sidewalls shieldthe well regions directly underneath them, these well regions are notdoped during the implantation process with the result that separatesource and drain regions and separate diffusion regions 560, 562 areformed. Holes are then made in the insulating layer and contacts areformed to the source and drain regions 510, 510A, 520, 520A and thediffusion regions 560, 562. Advantageously, the N-type diffusion regions560, 562 may be formed at the same time as the same process is used toform other N-type regions, such as source and drain regions, elsewhereon the integrated circuit; and similarly, the P-type process used toform the P-type source and drain regions 510, 510A, 520, 520A may beused to form P-type diffusion regions elsewhere on the integratedcircuit.

FIG. 7 is a top view of a second illustrative embodiment of theinvention. Structure 700 includes source and drain regions 710, 720formed in a well (not shown) with a polysilicon gate finger 740 formedon a dielectric layer (not shown) on the surface of the well. Theseelements will be recognized as forming a MOS transistor; but it will beunderstood that the MOS transistor is only illustrative of any activedevice that may be used in the practice of the invention. A second MOStransistor is formed on the right-hand side of FIG. 7 and includes thesame elements bearing the same numbers followed by the suffix A. Thewell is formed in a semiconductor substrate (not shown); and thecross-section of the active device, well and substrate of the embodimentof FIG. 7 is similar to the cross-section of the active device, well 630and substrate 605 of FIG. 6.

Structure 700 further includes diffusion regions 760, 762, 764 that makeohmic contact with well 730, at least two floating polysilicon gatefingers 782, 784 between diffusion regions 760, 762 and ohmic contacts(or taps) 715 to source region 710, ohmic contacts 725 to drain region720, and ohmic contacts 765, 767 to diffusion regions 760, 762. Nocontacts are made to diffusion region 764 with the result that region764 is left floating. The diffusion regions 760, 762, and contacts ortaps 765, 767 constitute the well strap. A STI region 750 surrounds theactive devices and the well strap. As shown in FIG. 7, taps 765 and taps767 are on opposite sides of floating gate fingers 782, 784.

Structure 700 further comprises dummy polysilicon gate fingers 775, 777located on opposite sides of the active device and above portions of theSTI region 750. As a result, the well strap is separated from the activedevice by only one length of the dummy polysilicon gate finger, therebyreducing the distance between the active device and the diffusion regioncompared to prior art structures.

The process for forming structure 700 and the resulting structuralcross-section are substantially the same as those of structure 500except that two floating polysilicon gate fingers 782, 784 are usedinstead of a single polysilicon gate finger 580 with the result thatthree diffusion regions 760, 762, 764 are formed instead of two.

FIG. 8 is a top view of a third illustrative embodiment of theinvention. Structure 800 includes source and drain regions 810, 820formed in a well (not shown) with a polysilicon gate finger 840 formedon a dielectric layer (not shown) on the surface of the well. Theseelements will be recognized as forming a MOS transistor; but it will beunderstood that the transistor is only illustrative of any active devicethat may be used in the practice of the invention. A second MOStransistor is formed on the right-hand side of FIG. 8 and includes thesame elements bearing the same numbers followed by the suffix A. Again,the well is formed in a semiconductor substrate (not shown); and thecross-section of the active device, well and substrate of the embodimentof FIG. 8 is similar to the cross-section of the active device, well 630and substrate 605 of FIG. 6.

Structure 800 further includes diffusion regions 860, 862 that makeohmic contact with well 830, a floating polysilicon gate finger 880between diffusion regions 860, 862 and ohmic contacts (or taps) 815 tosource region 810, ohmic contacts 825 to drain region 820, and ohmiccontacts 865 to diffusion region 860. As shown in FIG. 8, the contacts865 to diffusion region are located on only one side of the floatingpolysilicon gate finger 880 with the result that diffusion region 862 isleft floating. The diffusion region 860 and contacts or taps 865constitute the well strap. A STI region 850 surrounds the active devicesand the diffusion regions.

Structure 800 further comprises dummy polysilicon gate fingers 871, 872located on opposite sides of the active device and above the diffusionregions. As a result, the well strap is separated from the active deviceby only one length of the dummy polysilicon gate finger; and the size ofdiffusion region 862 is reduced by eliminating the taps on one side ofthe floating gate finger.

The process for forming structure 800 and the resulting structuralcross-section are substantially the same as those of structure 500except that contacts to the diffusion region are formed on only one sideof the floating polysilicon gate finger 880.

FIG. 9 is a top view of a fourth illustrative embodiment of theinvention. Structure 900 includes source and drain regions 910, 920formed in a well (not shown) with a polysilicon gate finger 940 formedon a dielectric layer (not shown) on the surface of the well. Theseelements will be recognized as forming a MOS transistor; but it will beunderstood that the MOS transistor is only illustrative of any activedevice that may be used in the practice of the invention. A second MOStransistor is formed on the right-hand side of FIG. 9 and includes thesame elements bearing the same numbers followed by the suffix A. Again,the well is formed in a semiconductor substrate (not shown); and thecross-section of the active device, well and substrate of the embodimentof FIG. 9 is similar to the cross-section of the active device, well 630and substrate 605 of FIG. 6.

Structure 900 further includes diffusion regions 960, 962, 964 that makeohmic contact with well 930, at least two floating polysilicon gatefingers 982, 984 between diffusion regions 960, 962, 964, and ohmiccontacts (or taps) 915 to source region 910, ohmic contacts 925 to drainregion 920, and ohmic contacts 965 to diffusion region 960. As shown inFIG. 9, the contacts 965 to diffusion region 960 are located on only oneside of the floating polysilicon gate fingers 982, 984 with the resultthat diffusion regions 962, 964 are left floating. The diffusion region960 and contacts or taps 965 constitute the well strap. A STI region 950surrounds the active devices and the well strap.

Structure 900 further comprises dummy polysilicon gate fingers 971, 972located on opposite sides of the active device and above portions of theSTI regions. As a result, the well strap is separated from the activedevice by only one length of the dummy polysilicon gate finger; and thesize of the diffusion region is reduced by eliminating the contacts onone side.

The process for forming structure 900 and the resulting structuralcross-section are substantially the same as those of structure 700except that contacts to the diffusion region are made on only one sideof the floating polysilicon gate fingers 982, 984.

As will be apparent to those skilled in the art, numerous variations maybe practiced within the spirit and scope of the present invention. Forexample, the well and diffusion region can be either a P-type well anddiffusion region or an N-type well and diffusion region. If the activedevice is a transistor, it can be an NMOS transistor in a P-well or aPMOS transistor in an N-well. Other active devices may also be used inthe practice of the invention. For purposes of illustration, thecontacts or taps have been depicted as a pair of contacts; but theinvention may be practiced with a single contact or with more than twocontacts. Other modifications will be apparent to those skilled in theart.

What is claimed is:
 1. A method comprising: forming a well in asemiconductor substrate; forming a first active device in the well;forming a strap in the well and separated from the first active device,wherein the strap comprises first and second diffusion regions onopposite sides of a floating polysilicon finger, and wherein the strapfurther comprises first and second taps that connect respectively to thefirst and second diffusion regions on opposite sides of the floatingpolysilicon finger; forming a first dummy polysilicon finger between thefirst active device and the strap; forming a second active device in thewell on an opposite side of the strap from the first active device; andforming a second dummy polysilicon finger between the second activedevice and the strap.
 2. The method of claim 1, wherein the first activedevice is an MOS transistor.
 3. The method of claim 1 furthercomprising: forming a shallow trench isolation region that surrounds thefirst active device and the strap.
 4. The method of claim 3, wherein theshallow trench isolation region surrounds the second active device. 5.The method of claim 1, wherein the first active device comprises a firstpolysilicon gate finger, and wherein the second active device comprisesa second polysilicon gate finger.
 6. The method of claim 5, wherein thefirst active device further comprises first source and drain regions,and wherein the second active device further comprises second source anddrain regions.
 7. The method of claim 1 further comprising: forming ashallow trench isolation region, wherein the first and second dummypolysilicon fingers are located above portions of the shallow trenchisolation region.
 8. The method of claim 1, wherein the well is dopedwith N-type dopants.
 9. The method of claim 2, wherein the second activedevice is an MOS transistor.
 10. A method comprising: forming a well ina semiconductor substrate; forming a first active device in the well;forming a strap in the well and separated from the first active device,wherein the strap comprises a diffusion region, a floating polysiliconfinger, and a tap connecting to the diffusion region on one side of thefloating polysilicon finger; forming a first dummy polysilicon finger onthe well between the first active device and the strap; forming a secondactive device in the well on an opposite side of the strap from thefirst active device; and forming a second dummy polysilicon finger onthe well between the second active device and the strap.
 11. The methodof claim 10, wherein the first active device is an MOS transistor. 12.The method of claim 10, wherein a plurality of taps connect to thediffusion region.
 13. The method of claim 10 further comprising: forminga shallow trench isolation region surrounding the first active deviceand the strap.
 14. The method of claim 13, wherein the shallow trenchisolation region surrounds the second active device.
 15. The method ofclaim 10, wherein the first active device comprises a first polysilicongate finger, and wherein the second active device comprises a secondpolysilicon gate finger.
 16. The method of claim 15, wherein the firstactive device further comprises first source and drain regions, andwherein the second active device further comprises second source anddrain regions.
 17. The method of claim 10 further comprising: forming ashallow trench isolation region, wherein the first and second dummypolysilicon fingers are located above portions of the shallow trenchisolation region.
 18. The method of claim 11, wherein the second activedevice is an MOS transistor.